This invention relates to removing contaminants from an olefin stream. In particular, this invention is to a method of removing water and/or oxygenated hydrocarbons from an olefin stream.
Olefins, particularly C2 and C3 olefins, are desirable as a feed source for making derivative products such as oligomers, e.g., higher olefins, and polymers such as polyethylene and polypropylene. Olefin feed sources have traditionally been produced by cracking petroleum feedstocks.
Oxygenates, however, are becoming an alternative feedstock for making olefins, particularly large quantities of ethylene and propylene for the production of higher olefins and plastic materials. In general, the olefins are formed by contacting the oxygenate components with a molecular sieve catalyst to catalytically convert the oxygenates to olefins.
For example, U.S. Pat. No. 4,499,327, discloses a process for making olefins from methanol using any of a variety of silicoaluminophosphate (SAPO) molecular sieve catalysts. The process is carried out at a temperature between 300xc2x0 C. and 500xc2x0 C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hrxe2x88x921. The process is highly selective for making ethylene and propylene.
U.S. Pat. No. 6,121,504 also discloses a method of making olefin product from oxygenate feed using molecular sieve catalysts. Water and other unwanted by-products are removed from the olefin product by contacting with a quench medium. After contacting with the quench medium, a light product fraction is obtained which comprises the desired olefins, but also includes dimethylether, methane, CO, CO2, ethane, propane, and other minor components such as water and unreacted oxygenate feedstock.
In order to further process olefins made from oxygenates, it is often necessary to reduce or remove the amount of oxygenated hydrocarbon by-products that are present in the recovered olefin product. This is because derivative manufacturing processes may use catalysts that are quite sensitive to the presence of oxygenated hydrocarbons. For example, oxygenated hydrocarbon components such as dimethylether and acetaldehyde have been found to act as poisons to certain higher olefin forming catalysts and polyolefin forming catalysts. Therefore, it is desirable to find efficient methods for removing oxygenated hydrocarbons from olefin streams which are predominantly comprised of ethylene and/or propylene.
U.S. Pat. No. 4,864,071 discloses a process for separating an unsaturated hydrocarbon from a gas mixture. The unsaturated hydrocarbons are produced by pyrolysis of saturated hydrocarbons such as petroleum fractions. The process includes contacting the gas mixture with a solid adsorbent made from a silver or copper halide and polystyrene.
U.S. Pat. No. 6,037,516 discloses a method for removing oxygenate impurities such as formaldehyde, acetaldehyde, propionaldehyde, and methyl formate from a propylene stream, which is a by-product stream of an alkylene oxide manufacturing process. The method of removing the impurities includes treating the propylene stream sequentially with aqueous sodium bisulfite and aqueous sodium hydroxide, accompanied by separation of the oxygenate impurity reaction products using coalescers.
U.S. Pat. No. 6,124,517 discloses a method for purifying olefins produced by thermal cracking of hydrocarbon feedstocks. The method uses heterogeneous adsorbents to remove acetylenic impurities and saturated hydrocarbon gases. The adsorbents comprise a metal supported on a high surface area carrier.
U.S. Pat. No. 4,513,156 discloses a method of removing oxygenated hydrocarbons from olefinic feedstocks produced from Fischer-Tropsch conversion of synthesis gas. The method uses water as an adsorbent, and is particularly useful where the olefinic feed is rich in C3+ mono-olefins and contains a minor amount Of C2-C6 oxygenates, and the olefin feed is used for the manufacture of higher olefins.
Additional methods are sought for removing oxygenated hydrocarbon, as well as water, contaminants from olefin streams. In particular, more desirable methods are sought for removing water and/or oxygenated hydrocarbons from ethylene and/or propylene containing streams, particularly such streams made from an oxygenate to olefin reaction process.
This invention provides an effective method for removing oxygen containing compounds such as water and/or oxygenated hydrocarbons from olefin streams. The method is particularly effective in removing water and/or oxygenated hydrocarbons from olefin streams produced in oxygenate to olefin reaction processes.
In one embodiment, the method of removing water and/or oxygenated hydrocarbons from an olefin stream comprises contacting an oxygenate with an olefin forming catalyst to form an olefin stream, wherein the olefin stream comprises olefin and oxygenated hydrocarbon. The olefin stream is then contacted with an absorbent in an absorption system to absorb the water and/or oxygenated hydrocarbon from the olefin stream, wherein the absorbent is selected from the group consisting of a polyol, amine, amide, nitrile, heterocyclic nitrogen containing compound, and mixtures thereof.
In another embodiment, the method includes removing an olefin stream from the absorption system. In yet another embodiment, the olefin stream removed from the absorption system contains at least 50 wt % less water and/or oxygenated hydrocarbon than the olefin stream formed by contacting the oxygenate with the catalyst.
Additional water and/or oxygenated hydrocarbon is removed in embodiments of the invention which include an adsorbent system. In these embodiments, an olefin product stream is formed which contains not greater than 1 wppm water.
In another embodiment of the invention, water and/or oxygenated hydrocarbons are removed from an olefin stream containing at least 50 wt % ethylene and propylene, not greater than 20 wt % water, and not greater than 15 wt % oxygenated hydrocarbon. The water and/or oxygenated hydrocarbons are removed by contacting the olefin stream with an absorbent, wherein the absorbent is selected from the group consisting of a polyol, amine, amide, nitrile, heterocyclic nitrogen containing compound, and mixtures thereof.
Examples of polyol, amine, amide, nitrile, heterocyclic nitrogen containing compounds which can be used include ethylene glycol, diethylene glycol, triethylene glycol, ethanolamine, diethanolamine, triethylamine, hindered cyclic amines, acetonitrile, n-methylpyrrolidone, and dimethyl formamide, as well as mixtures of any two or more of these compounds. Olefins treated in accordance with this invention are particularly suitable for use as feedstock for making polyolefins.